Color display device

ABSTRACT

The present invention provides a color display device in which each pixel can display four high quality color states. More specifically, an electrophoretic fluid is provided which comprises four types of particles, dispersed in a solvent or solvent mixture. The fluid may further comprise substantially uncharged neutral buoyancy particles.

This application claims the benefit of U.S. Provisional Application Nos. 61/824,887, filed May 17, 2013; 61/893,831, filed Oct. 21, 2013; and 61/974,858, filed Apr. 3, 2014. The contents of the above-identified applications are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention is directed to a color display device in which each pixel can display four high quality color states, an electrophoretic fluid for such an electrophoretic display and driving methods for such a display device.

BACKGROUND OF THE INVENTION

In order to achieve a color display, color filters are often used. The most common approach is to add color filters on top of black/white sub-pixels of a pixellated display to display the red, green and blue colors. When a red color is desired, the green and blue sub-pixels are turned to the black state so that the only color displayed is red. When the black state is desired, all three-sub-pixels are turned to the black state. When the white state is desired, the three sub-pixels are turned to red, green and blue, respectively, and as a result, a white state is seen by the viewer.

The biggest disadvantage of such a technique is that since each of the sub-pixels has a reflectance of about one third of the desired white state, the white state is fairly dim. To compensate this, a fourth sub-pixel may be added which can display only the black and white states, so that the white level is doubled at the expense of the red, green or blue color level (where each sub-pixel is only one fourth of the area of the pixel). Brighter colors can be achieved by adding light from the white pixel, but this is achieved at the expense of color gamut to cause the colors to be very light and unsaturated. A similar result can be achieved by reducing the color saturation of the three sub-pixels. Even with this approach, the white level is normally substantially less than half of that of a black and white display, rendering it an unacceptable choice for display devices, such as e-readers or displays that need well readable black-white brightness and contrast.

SUMMARY OF THE INVENTION

One aspect of the present invention is directed to a display layer comprising an electrophoretic medium and having first and second surfaces on opposed sides thereof, the electrophoretic medium comprising a first type of positive particles, a first type of negative particles, a second type of positive particles and a second type of negative particles, all dispersed in a solvent or solvent mixture, the four type of particles having respectively optical characteristics differing from one another, such that:

(a) application of an electric field which has the same polarity as the first type of positive particles will cause the optical characteristics of the first type of positive particles to be displayed at the first surface; or

(b) application of an electric field which has the same polarity as the first type of negative particles will cause the optical characteristic of the first type of negative particles to be displayed at the first surface; or

(c) once the optical characteristic of the first type of positive particles is displayed at the first surface, application of an electric field which has the same polarity as the second type of negative particles, but is not strong enough to overcome the attraction force between the first type of positive particles and the first type of negative particles, but sufficient to overcome the attraction forces between other oppositely charged particles will cause the optical characteristic of the second type of negative particles to be displayed at the first surface; or

(d) once the optical characteristic of the first type of negative particles is displayed at the first surface, application of an electric field which has the same polarity as the second type of positive particles, but is not strong enough to overcome the attraction force between the first type of positive particles and the first type of negative particles, but sufficient to overcome the attraction forces between other oppositely charged particles will cause the optical characteristic of the second type of positive particles to be displayed at the first surface.

In one embodiment, the first type of positive particles is black particles, the first type of negative particles is yellow particles, the second type of positive particles is the red particles and the second type of negative particles is the white particles.

In one embodiment, the charges of the first type of positive particles and the first type of negative particles are higher than the second type of positive particles and the second type of negative particles.

In one embodiment, the charges of the lower charged particles are less than 50% of the charges of the higher charged particles. In one embodiment, the charges of the lower charged particles are 5% to 30% of the charges of the higher charged particles. In one embodiment, the charges of the lower charged particles are less than 75% of the charges of the higher charged particles. In one embodiment, the charges of the lower charged particles are 15% to 55% of the charges of the higher charged particles.

In one embodiment, the electrophoretic medium further comprising substantially uncharged neutral buoyancy particles. In one embodiment, the neutral buoyancy particles are non-charged.

Another aspect of the present invention is directed to a driving method for an electrophoretic fluid comprising four types of charged pigment particles dispersed in a solvent or solvent mixture, wherein the four types of charged pigment particles are high positive charged pigment particles, high negative charged pigment particles, low positive charged pigment particles and low negative charged particles, which method comprises

(a) driving a pixel to the color state of one of the low charged particles; followed by

(b) driving the pixel to the color state of high charged particles, wherein the low charged particles and the high charged particles carry opposite charge polarities.

In one embodiment, the method further comprises a shaking waveform.

In one embodiment of the driving method, the high positive charged particles are black. In another embodiment, the high negative charged particles are yellow. In a further embodiment, the low positive charged particles are red. In yet a further embodiment, the low negative charged particles are white.

A further aspect of the invention is directed to a driving method for an electrophoretic fluid comprising four types of charged pigment particles dispersed in a solvent or solvent mixture, wherein the four types of charged pigment particles are high positive charged pigment particles, high negative charged pigment particles, low positive charged pigment particles and low negative charged particles, which method comprises

-   -   (a) applying a shaking waveform;     -   (b) applying a high driving voltage having the same polarity as         one type of high charged pigment particles to drive to a color         state of the high charged pigment particles;     -   (c) applying a low driving voltage having the same polarity as         one type of low charged pigment particles to drive to a color         state of low charged pigment particles; and     -   (d) applying a high driving voltage having the same polarity as         the high charged pigment particles to drive to a color state of         the high charged pigment particles;         wherein the high charged pigment particles and the low charged         pigment particles are oppositely charged and the driving method         is DC balanced.

In yet a further aspect of the invention is directed to an electrophoretic fluid comprising four types of charged pigment particles dispersed in a solvent or solvent mixture, wherein the four types of charged pigment particles are high positive charged pigment particles, high negative charged pigment particles, low positive charged pigment particles and low negative charged particles and the low charged particles have a charge intensity which is less than 75% of the charge intensity of the high charged particles.

In one embodiment, the low positive charged particles have a charge intensity which is less than 50% of the charge intensity of the high positive charged particles and the low negative charged particles have a charge intensity which is less than 75% of the charge intensity of the high negative charged particles.

In one embodiment, the fluid further comprises substantially uncharged neutral buoyancy particles, which may be non-charged.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a display layer which can display four different color states.

FIGS. 2-1 to 2-3 illustrate an example of the present invention.

FIG. 3 demonstrates display cells unaligned with pixel electrodes.

FIGS. 4A and 4B illustrate driving methods of the present invention.

FIG. 5 shows a shaking waveform which may be incorporated into driving sequences.

FIGS. 6A and 6B show example waveforms for driving the display device of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The electrophoretic fluid of present invention comprises two pairs of oppositely charged particles. The first pair consists of a first type of positive particles and a first type of negative particles and the second pair consists of a second type of positive particles and a second type of negative particles.

In the two pairs of oppositely charged particles, one pair carries a stronger charge than the other pair. Therefore the four types of particles may also be referred to as high positive particles, high negative particles, low positive particles and low negative particles.

As an example shown in FIG. 1, the black particles (K) and yellow particles (Y) are the first pair of oppositely charged particles, and in this pair, the black particles are the high positive particles and the yellow particles are the high negative particles. The red particles (R) and the white particles (W) are the second pair of oppositely charged particles and in this pair, the red particles are the low positive particles and the white particles are the low negative particles.

In another example not shown, the black particles may be the high positive particles; the yellow particles may be the low positive particles; the white particles may be the low negative particles and the red particles may be the high negative particles.

In addition, the color states of the four types of particles may be intentionally mixed. For example, because yellow pigment by nature often has some greenish tint and if a better yellow color state is desired, yellow particles and red particles may be used where both types of particles carry the same charge polarity and the yellow particles are higher charged than the red particles. As a result, at the yellow state, there will be a small amount of the red particles mixed with the greenish yellow particles to cause the yellow state to have better color purity.

It is understood that the scope of the invention broadly encompasses particles of any colors as long as the four types of particles have visually distinguishable colors.

For the white particles, they may be formed from an inorganic pigment, such as TiO₂, ZrO₂, ZnO, Al₂O₃, Sb₂O₃, BaSO₄, PbSO₄ or the like.

For the black particles, they may be formed from CI pigment black 26 or 28 or the like (e.g., manganese ferrite black spinel or copper chromite black spinel) or carbon black.

Particles of other colors are independently of a color such as red, green, blue, magenta, cyan or yellow. The pigments for color particles may include, but are not limited to, CI pigment PR 254, PR122, PR149, PG36, PG58, PG7, PB28, PB15:3, PY83, PY138, PY150, PY155 or PY20. Those are commonly used organic pigments described in color index handbooks, “New Pigment Application Technology” (CMC Publishing Co, Ltd, 1986) and “Printing Ink Technology” (CMC Publishing Co, Ltd, 1984). Specific examples include Clariant Hostaperm Red D3G 70-EDS, Hostaperm Pink E-EDS, PV fast red D3G, Hostaperm red D3G 70, Hostaperm Blue B2G-EDS, Hostaperm Yellow H4G-EDS, Novoperm Yellow HR-70-EDS, Hostaperm Green GNX, BASF Irgazine red L 3630, Cinquasia Red L 4100 HD, and Irgazin Red L 3660 HD; Sun Chemical phthalocyanine blue, phthalocyanine green, diarylide yellow or diarylide AAOT yellow.

The color particles may also be inorganic pigments, such as red, green, blue and yellow. Examples may include, but are not limited to, CI pigment blue 28, CI pigment green 50 and CI pigment yellow 227.

In addition to the colors, the four types of particles may have other distinct optical characteristics, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.

A display layer utilizing the display fluid of the present invention has two surfaces, a first surface (13) on the viewing side and a second surface (14) on the opposite side of the first surface (13). The display fluid is sandwiched between the two surfaces. On the side of the first surface (13), there is a common electrode (11) which is a transparent electrode layer (e.g., ITO), spreading over the entire top of the display layer. On the side of the second surface (14), there is an electrode layer (12) which comprises a plurality of pixel electrodes (12 a).

The pixel electrodes are described in U.S. Pat. No. 7,046,228, the content of which is incorporated herein by reference in its entirety. It is noted that while active matrix driving with a thin film transistor (TFT) backplane is mentioned for the layer of pixel electrodes, the scope of the present invention encompasses other types of electrode addressing as long as the electrodes serve the desired functions.

Each space between two dotted vertical lines in FIG. 1 denotes a pixel. As shown, each pixel has a corresponding pixel electrode. An electric field is created for a pixel by the potential difference between a voltage applied to the common electrode and a voltage applied to the corresponding pixel electrode.

The percentages of the four types of particles in the fluid may vary. For example, in a fluid having black/yellow/red/white particles, the black particle may take up 0.1% to 10%, preferably 0.5% to 5%, by volume of the electrophoretic fluid; the yellow particle may take up 1% to 50%, preferably 5% to 15%, by volume of the fluid; and each type of the red and white particles may take up 2% to 20%, preferably 4% to 10%, by volume of the fluid.

The solvent in which the four types of particles are dispersed is clear and colorless. It preferably has a low viscosity and a dielectric constant in the range of about 2 to about 30, preferably about 2 to about 15 for high particle mobility. Examples of suitable dielectric solvent include hydrocarbons such as isopar, decahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils, paraffin oil, silicon fluids, aromatic hydrocarbons such as toluene, xylene, phenylxylylethane, dodecylbenzene or alkylnaphthalene, halogenated solvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene, dichlorobenzotrifluoride, 3,4,5-trichlorobenzotri fluoride, chloropentafluoro-benzene, dichlorononane or pentachlorobenzene, and perfluorinated solvents such as FC-43, FC-70 or FC-5060 from 3M Company, St. Paul Minn., low molecular weight halogen containing polymers such as poly(perfluoropropylene oxide) from TCI America, Portland, Oreg., poly(chlorotrifluoro-ethylene) such as Halocarbon Oils from Halocarbon Product Corp., River Edge, N.J., perfluoropolyalkylether such as Galden from Ausimont or Krytox Oils and Greases K-Fluid Series from DuPont, Del., polydimethylsiloxane based silicone oil from Dow-corning (DC-200).

In one embodiment, the charge carried by the “low charge” particles may be less than about 50%, preferably about 5% to about 30%, of the charge carried by the “high charge” particles. In another embodiment, the “low charge” particles may be less than about 75%, or about 15% to about 55%, the charge carried by the “high charge” particles. In a further embodiment, the comparison of the charge levels as indicated applies to two types of particles having the same charge polarity.

The charge intensity may be measured in terms of zeta potential. In one embodiment, the zeta potential is determined by Colloidal Dynamics AcoustoSizer IIM with a CSPU-100 signal processing unit, ESA EN# Attn flow through cell (K:127). The instrument constants, such as density of the solvent used in the sample, dielectric constant of the solvent, speed of sound in the solvent, viscosity of the solvent, all of which at the testing temperature (25° C.) are entered before testing. Pigment samples are dispersed in the solvent (which is usually a hydrocarbon fluid having less than 12 carbon atoms), and diluted to between 5-10% by weight. The sample also contains a charge control agent (Solsperse 17000®, available from Lubrizol Corporation, a Berkshire Hathaway company; “Solsperse” is a Registered Trade Mark), with a weight ratio of 1:10 of the charge control agent to the particles. The mass of the diluted sample is determined and the sample is then loaded into the flow through cell for determination of the zeta potential.

The magnitudes of the “high positive” particles and the “high negative” particles may be the same or different. Likewise, the magnitudes of the “low positive” particles and the “low negative” particles may be the same or different.

It is also noted that in the same fluid, the two pairs of high-low charge particles may have different levels of charge differentials. For example, in one pair, the low positively charged particles may have a charge intensity which is 30% of the charge intensity of the high positively charged particles and in another pair, the low negatively charged particles may have a charge intensity which is 50% of the charge intensity of the high negatively charged particles.

It is also noted that the four types of particles may have different particle sizes. For example, the smaller particles may have a size which ranges from about 50 nm to about 800 nm. The larger particles may have a size which is about 2 to about 50 times, and more preferably about 2 to about 10 times, the sizes of the smaller particles.

The following is an example illustrating the present invention.

EXAMPLE 1

This example is demonstrated in FIG. 2. The high positive particles are of the black color (K); the high negative particles are of a yellow color (Y); the low positive particles are of a red color (R); and the low negative particles are of a white color (W).

In FIG. 2( a), when a high negative voltage potential difference (e.g., −15V) is applied to a pixel for a time period of sufficient length, an electric field is generated to cause the yellow particles (Y) to be pushed to the common electrode (21) side and the black particles (K) pulled to the pixel electrode (22 a) side. The red (R) and white (W) particles, because they carry weaker charges, move slower than the higher charged black and yellow particles and as a result, they stay in the middle of the pixel, with white particles above the red particles. In this case, a yellow color is seen at the viewing side.

In FIG. 2( b), when a high positive voltage potential difference (e.g., +15V) is applied to the pixel for a time period of sufficient length, an electric field of an opposite polarity is generated which causes the particle distribution to be opposite of that shown in FIG. 2( a) and as a result, a black color is seen at the viewing side.

In FIG. 2( c), when a lower positive voltage potential difference (e.g., +3V) is applied to the pixel of FIG. 2( a) (that is, driven from the yellow state) for a time period of sufficient length, an electric field is generated to cause the yellow particles (Y) to move towards the pixel electrode (22 a) while the black particles (K) move towards the common electrode (21). However, when they meet in the middle of the pixel, they stop moving and remain there because the electric field generated by the low driving voltage is not strong enough to overcome the strong attraction between them. On the other hand, the electric field generated by the low driving voltage is sufficient to separate the weaker charged white and red particles to cause the low positive red particles (R) to move all the way to the common electrode (21) side (i.e., the viewing side) and the low negative white particles (W) to move to the pixel electrode (22 a) side. As a result, a red color is seen. It is also noted that in this figure, there are also attraction forces between weaker charged particles (e.g., R) with stronger charged particles of opposite polarity (e.g., Y). However, these attraction forces are not as strong as the attraction forces between two types of stronger charged particles (K and Y) and therefore they can be overcome by the electric field generated by the low driving voltage. In other words, weaker charged particles and the stronger charged particles of opposite polarity can be separated.

In FIG. 2( d), when a lower negative voltage potential difference (e.g., −3V) is applied to the pixel of FIG. 2( b) (that is, driven from the black state) for a time period of sufficient length, an electric field is generated which causes the black particles (K) to move towards the pixel electrode (22 a) while the yellow particles (Y) move towards the common electrode (21). When the black and yellow particles meet in the middle of the pixel, they stop moving and remain there because the electric field generated by the low driving voltage is not sufficient to overcome the strong attraction between them. At the same time, the electric field generated by the low driving voltage is sufficient to separate the white and red particles to cause the low negative white particles (W) to move all the way to the common electrode side (i.e., the viewing side) and the low positive red particles (R) move to the pixel electrode side. As a result, a white color is seen. It is also noted that in this figure, there are also attraction forces between weaker charged particles (e.g., W) with stronger charged particles of opposite polarity (e.g., K). However, these attraction forces are not as strong as the attraction forces between two types of stronger charged particles (K and Y) and therefore they can be overcome by the electric field generated by the low driving voltage. In other words, weaker charged particles and the stronger charged particles of opposite polarity can be separated.

Although in this example, the black particles (K) is demonstrated to carry a high positive charge, the yellow particles (Y) carry a high negative charge, the red (R) particles carry a low positive charge and the white particles (W) carry a low negative charge, in practice, the particles carry a high positive charge, or a high negative charge, or a low positive charge or a low negative charge may be of any colors. All of these variations are intended to be within the scope of this application.

It is also noted that the lower voltage potential difference applied to reach the color states in FIGS. 2( c) and 2(d) may be about 5% to about 50% of the full driving voltage potential difference required to drive the pixel from the color state of high positive particles to the color state of the high negative particles, or vice versa.

The electrophoretic fluid as described above is filled in display cells. The display cells may be microcups as described in U.S. Pat. No. 6,930,818, the content of which is incorporated herein by reference in its entirety. The display cells may also be other types of micro-containers, such as microcapsules, microchannels or equivalents, regardless of their shapes or sizes. All of these are within the scope of the present application.

As shown in FIG. 3, the display cells (30), in the present invention, and the pixel electrodes (32 a) do not have to be aligned.

In a further aspect of the present invention, the fluid may further comprise substantially uncharged neutral buoyancy particles.

The term “substantially uncharged” refers to the particles which are either uncharged or carry a charge which is less than 5% of the average charge carried by the higher charged particles. In one embodiment, the neutral buoyancy particles are non-charged.

The term “neutral buoyancy” refers to particles which do not rise or fall with gravity. In other words, the particles would float in the fluid between the two electrode plates. In one embodiment, the density of the neutral buoyancy particles may be the same as the density of the solvent or solvent mixture in which they are dispersed.

The concentration of the substantially uncharged neutral buoyancy particles in the display fluid is preferably in the range of about 0.1 to about 10% by volume, more preferably in the range of about 0.1 to about 5% by volume.

The term “about” refers to a range which is ±10% of the indicated value.

The substantially uncharged neutral buoyancy particles may be formed from a polymeric material. The polymeric material may be a copolymer or a homopolymer.

Examples of the polymeric material for the substantially uncharged neutral buoyancy particles may include, but are not limited to, polyacrylate, polymethacrylate, polystyrene, polyaniline, polypyrrole, polyphenol and polysiloxane. Specific examples of the polymeric material may include, but are not limited to, poly(pentabromophenyl methacrylate), poly(2-vinylnapthalene), poly(naphthyl methacrylate), poly(alpha-methystyrene), poly(N-benzyl methacrylamide) and poly(benzyl methacrylate).

More preferably, the substantially uncharged neutral buoyancy particles are formed from a polymer which is not soluble in the solvent of the display fluid, and also has a high refractive index. In one embodiment, the refractive index of the substantially uncharged neutral buoyancy particles is different from that of the solvent or solvent mixture in which the particles are dispersed. However, typically the refractive index of the substantially uncharged neutral buoyancy particles is higher than that of the solvent or solvent mixture. In some cases, the refractive index of the substantially uncharged neutral buoyancy particles may be above 1.45.

In one embodiment, the materials for the substantially uncharged neutral buoyancy particles may comprise an aromatic moiety.

The substantially uncharged neutral buoyancy particles may be prepared from monomers through polymerization techniques, such as suspension polymerization, dispersion polymerization, seed polymerization, soap-free polymerization, emulsion polymerization or physical method, including inverse emulsification-evaporation process. The monomers are polymerized in the presence of a dispersant. The presence of the dispersant allows the polymer particles to be formed in a desired size range and the dispersant may also form a layer physically or chemically bonded to the surface of the polymer particles to prevent the particles from agglomeration.

The dispersant preferably has a long chain (of at least eight atoms), which may stabilize the polymer particles in a hydrocarbon solvent. Such dispersants may be an acrylate-terminated or vinyl-terminated macromolecule, which are suitable because the acrylate or vinyl group can co-polymerize with the monomer in the reaction medium.

One specific example of the dispersant is acrylate terminated polysiloxane (Gelest, MCR-M17, MCR-M22),

Another type of suitable dispersants is polyethylene macromonomers, as shown below: CH₃—[—CH₂—]_(n)—CH₂O—C(═O)—C(CH₃)═CH₂

The backbone of the macromonomer may be a polyethylene chain and the integer “n” may be 30-200. The synthesis of this type of macromonomers may be found in Seigou Kawaguchi et al, Designed Monomers and Polymers, 2000, 3, 263.

If the fluid system is fluorinated, the dispersants are then preferably also fluorinated.

Alternatively, the substantially uncharged neutral buoyancy particles may also be formed from a core particle coated with a polymeric shell and the shell may be formed, for example, from any of the polymeric material identified above.

The core particle may be of an inorganic pigment such as TiO₂, ZrO₂, ZnO, Al₂O—₃, CI pigment black 26 or 28 or the like (e.g., manganese ferrite black spinel or copper chromite black spinel), or an organic pigment such as phthalocyanine blue, phthalocyanine green, diarylide yellow, diarylide AAOT yellow, and quinacridone, azo, rhodamine, perylene pigment series from Sun Chemical, Hansa yellow G particles from Kanto Chemical, and Carbon Lampblack from Fisher, or the like.

In the case of core-shell substantially uncharged neutral buoyancy particles, they may be formed by a microencapsulation method, such as coacervation, interfacial polycondensation, interfacial cross-linking, in-suit polymerization or matrix polymerization.

The size of the substantially uncharged neutral buoyancy particles is preferably in the range of about 100 nanometers to about 5 microns.

In one embodiment of this aspect of the present invention, the substantially uncharged neutral buoyancy particles added to the fluid may have a color substantially the same visually to the color of one of the four types of charged particles. For example, in a display fluid, there may be charged black, yellow, red and white particles and substantially uncharged neutral buoyancy particles, and in this case, the substantially uncharged neutral buoyancy particles may be black, yellow, red or white.

In another embodiment, the substantially uncharged neutral buoyancy particles may have a color substantially different from the color of either one of the four types of charged particles.

The presence of the substantially uncharged neutral buoyancy particles in the fluid increases reflection of incident light, thus also improving the contrast ratio, especially if they are formed from a reflective material.

The image stability may also be improved by the addition of the substantially uncharged neutral buoyancy particles in the four particle fluid system. The substantially uncharged neutral buoyancy particles can fill in the gaps resulted from the charged particles being over packed on the surface of an electrode under an electrical field, thus preventing the charged particles from settling due to the gravitational force.

In addition, if the substantially uncharged neutral buoyancy particles are white, they may enhance the reflectivity of the display. If they are black, they may enhance the blackness of the display.

In any case, the substantially uncharged neutral buoyancy particles do not affect the driving behavior of the four types of charged particles in the fluid.

Ideally when a high positive driving voltage (e.g. +15V) is applied as shown in FIG. 2( b), the electric field generated would cause the high positive black particles to move towards the common electrode side (i.e., the viewing side) and the high negative yellow particles and the low negative white particles to move towards the non-viewing side, to show the black state. The low positive red particles would move towards the viewing side. But since the red particles carry a lower charge compared to the black particles, they move slower and as a result, the black color is seen at the viewing side. However, in practice, the black state achieved may have a reddish tint. This could be caused by some of the red particles becoming mixed with the black particles at the viewing side.

The present invention also provides driving methods which can resolve the unsatisfactory color issue. In one of the driving methods, a pixel is first driven towards the color state of one of the low charged particles before being driven towards the color state of high charged particles, wherein the low charged particles and the high charged particles carry opposite charge polarities.

For example, a pixel may be driven towards the black color state, according to the following steps:

-   -   a) driving first to the color state of the white particles (low         negative charged) by applying a low negative driving voltage;         and     -   b) driving towards the color state of the black particles (high         positive charged) by applying a high positive driving voltage.

This driving sequence is illustrated in FIG. 4A.

In step (a), once at the white state (e.g., FIG. 2( d)), the two types of “high charged” particles, black and yellow, will attract to each other to cause them to stay in the middle of the pixel and the low positive charged red pigment particles would move to be near or at the pixel electrode.

In step (b), the white and yellow particles are pushed to the pixel electrode side, and the low positive charged red particles are much less likely to show up at the viewing side. This sequence will result in a better quality of the black state.

In this driving method, a white color state is driven directly towards the black state without going through the red or yellow color state. It has also been found that higher the quality of the white state in step (a) will lead to a higher quality of the black state in step (b). The “higher quality of the white state” simply means a high L* value and low a* and b* values in the L*a*b* color system, for the white state.

A similar driving method may be applied to driving a pixel to the yellow state. The method will have the following steps:

-   -   a) driving first to the color state of the red particles (low         positive charged) by applying a low positive driving voltage;         and     -   b) driving towards the color state of the yellow particles (high         negative charged) by applying a high negative driving voltage.

This driving sequence is shown in FIG. 4B.

In this driving method, a red color state is driven directly towards the yellow state without going through the white or black color state. It has also been found that higher the quality of the red state in step (a) will lead to a higher quality of the yellow state in step (b). The “higher quality of the red state” simply means a high a* value in the L*a*b* color system, for the red state.

The driving method shown in FIGS. 4A and 4B may also be summarized as follows:

A driving method for driving a display layer which comprises an electrophoretic medium and has first and second surfaces on opposed sides thereof, the electrophoretic medium comprising a first type of positive particles, a first type of negative particles, a second type of positive particles and a second type of negative particles, all dispersed in a solvent or solvent mixture, the four type of particles having respectively optical characteristics differing from one another, which method comprises:

(a) applying an electric field which is not sufficient to overcome the attraction force between the first type of positive particles and the first type of negative particles and has the same polarity as the second type of positive or negative particles to cause the optical characteristics of the second type of positive or negative particles to be displayed at the first surface; and

(b) applying an electric field which is sufficient to overcome the attraction force between the first type of positive particles and the first type of negative particles and has the polarity opposite of the polarity of the electric field in step (a) to cause the optical characteristic of the first type of positive particles or the first type of negative particles to be displayed at the first surface.

In addition, to ensure both color brightness and color purity, a shaking waveform, prior to driving from one color state to another color state, may be used. The shaking waveform consists of repeating a pair of opposite driving pulses for many cycles. For example, the shaking waveform may consist of a +15V pulse for 20 msec and a −15V pulse for 20 msec and such a pair of pulses is repeated for 50 times. The total time of such a shaking waveform would be 2000 msec (see FIG. 5).

In practice, there may be at least 10 repetitions (i.e., ten pairs of positive and negative pulses).

The shaking waveform may be applied regardless of the optical state (black, white, red or yellow) prior to a driving voltage is applied. After the shaking waveform is applied, the optical state would not be a pure white, pure black, pure yellow or pure red. Instead, the color state would be from a mixture of the four types of pigment particles.

Each of the driving pulse in the shaking waveform is applied for not exceeding 50% (or not exceeding 30%, 10% or 5%) of the driving time required from the full black state to the full yellow state in the example. For example, if it takes 300 msec to drive a display device from a full black state to a full yellow state or vice versa, the shaking waveform may consist of positive and negative pulses, each applied for not more than 150 msec. In practice, it is preferred that the pulses are shorter.

In one embodiment, a shaking waveform may be applied prior to the driving sequence of FIG. 4A or FIG. 4B.

In another embodiment, a pixel may be:

-   -   (i) applied a shaking waveform;     -   (ii) driven to black (i.e., the first-time black state);     -   (iii) driven to white; and then     -   (iv) driven to black (i.e., the second-time black state).

In this sequence, step (ii) may be carried out according to FIG. 2( b); step (iii) may be carried out according to FIG. 2( d); and step (iv) may be carried out according to FIG. 4A.

An example waveform for this driving sequence is shown in FIG. 6A. In any of the driving sequences of the present invention, the waveforms are preferably DC balanced, that is, the average voltage applied across the display is substantially zero when integrated over a time period. In FIG. 6A, in the initial step as shown, a high negative driving voltage is applied to ensure DC balance of the entire waveform.

Similarly, both the shaking waveform and the method of FIG. 4B may be incorporated into a driving sequence:

-   -   (i) applied a shaking waveform;     -   (ii) driven to yellow (i.e., the first-time yellow state);     -   (iii) driven to red; and then     -   (iv) driven to yellow (i.e., the second-time yellow state).

In this sequence, step (ii) may be carried out according to FIG. 2( a); step (iii) may be carried out according to FIG. 2( c); and step (iv) may be carried out according to FIG. 4B.

An example waveform for this driving sequence is shown in FIG. 6B, which is also “DC balanced”.

In practice, the quality of the first-time color state (black or yellow) is usually inferior compared with the second-time color state (black or yellow).

While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation, materials, compositions, processes, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto. 

What is claimed is:
 1. A display layer comprising an electrophoretic medium, a first surface on a viewing side, and a second surface on the opposite of the first surface, the electrophoretic medium being sandwiched between the first and the second surfaces and comprising a first type of positive particles, a first type of negative particles, a second type of positive particles, and a second type of negative particles, all dispersed in a solvent or solvent mixture, the four type of particles having respectively optical characteristics differing from one another, such that: (a) application to a pixel of an electric field having the same polarity as the first type of positive particles will cause the optical characteristics of the first type of positive particles to be displayed at the first surface; or (b) application to a pixel of an electric field which has the same polarity as the first type of negative particles will cause the optical characteristic of the first type of negative particles to be displayed at the first surface; or (c) once a pixel is driven to display the optical characteristic of the first type of positive particles, application to a pixel of an electric field having the same polarity as the second type of negative particles, which electric field is not strong enough to overcome the attraction force between the first type of positive particles and the first type of negative particles, but sufficient to overcome the attraction forces between other oppositely charged particles, will cause the optical characteristic of the second type of negative particles to be displayed at the first surface; or (d) once a pixel is driven to display the optical characteristic of the first type of negative particles, application to a pixel of an electric field having the same polarity as the second type of positive particles, which electric field is not strong enough to overcome the attraction force between the first type of positive particles and the first type of negative particles, but sufficient to overcome the attraction forces between other oppositely charged particles, will cause the optical characteristic of the second type of positive particles to be displayed at the first surface.
 2. The layer of claim 1, wherein the first type of positive particles is black particles, the first type of negative particles is yellow particles, the second type of positive particles is the red particles and the second type of negative particles is the white particles.
 3. The layer of claim 1, wherein the charges of the first type of positive particles and the first type of negative particles are higher than the charges of the second type of positive particles and the second type of negative particles.
 4. The layer of claim 3, wherein the charges of the lower charged particles are less than 50% of the charges of the higher charged particles, wherein the lower charged particles and the higher charged particles have the same charge polarity.
 5. The layer of claim 3, wherein the charges of the lower charged particles are 5% to 30% of the charges of the higher charged particles, wherein the lower charged particles and the higher charged particles have the same charge polarity.
 6. The layer of claim 3, wherein the charges of the lower charged particles are less than 75% of the charges of the higher charged particles, wherein the lower charged particles and the higher charged particles have the same charge polarity.
 7. The layer of claim 3, wherein the charges of the lower charged particles are 15% to 55% of the charges of the higher charged particles, wherein the lower charged particles and the higher charged particles have the same charge polarity.
 8. The layer of claim 1, wherein the electrophoretic medium further comprising substantially uncharged neutral buoyancy particles.
 9. The layer of claim 8, wherein the neutral buoyancy particles are non-charged.
 10. A driving method for an electrophoretic fluid comprising four types of charged pigment particles dispersed in a solvent or solvent mixture, wherein the four types of charged pigment particles are high positive charged pigment particles, high negative charged pigment particles, low positive charged pigment particles and low negative charged particles; the method comprises: (a) driving a pixel to the color state of one of the low charged particles, followed by (b) driving the pixel to the color state of high charged particles; wherein the low charged particles of (a) and the high charged particles of (b) carry opposite charge polarities.
 11. The driving method of claim 10, further comprising a shaking waveform.
 12. The driving method of claim 10, wherein the high positive charged particles are black.
 13. The driving method of claim 10, wherein the high negative charged particles are yellow.
 14. The driving method of claim 10, wherein the low positive charged particles are red.
 15. The driving method of claim 10, wherein the low negative charged particles are white.
 16. The driving method of claim 10, wherein the high positive charged particles are black, the high negative charged particles are yellow, the low positive charged particles are red, and the low negative charged particles are white.
 17. A driving method for an electrophoretic fluid comprising four types of charged pigment particles dispersed in a solvent or solvent mixture, wherein the four types of charged pigment particles are high positive charged pigment particles, high negative charged pigment particles, low positive charged pigment particles and low negative charged particles; the method comprises: (a) applying a shaking waveform to a pixel, (b) applying a high driving voltage having the same polarity as one type of high charged pigment particles to drive the pixel to color state of the type of high charged pigment particles, (c) applying a low driving voltage having the same polarity as one type of low charged pigment particles to drive the pixel to color state of the type of low charged pigment particles; and (d) applying a high driving voltage having the same polarity as the type of high charged pigment particles to drive the pixel to color state of the type of high charged pigment particles; wherein the type of high charged pigment particles of (b) and (d) and the type of low charged pigment particles of (c) are oppositely charged and the driving method is DC balanced. 